linux-kernel - Re: [PATCH v2] spi: qup: Add DMA capabilities

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Date:	Tue, 24 Feb 2015 18:08:54 +0200
From:	Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:	Mark Brown <broonie@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	devicetree@...r.kernel.org, linux-spi@...r.kernel.org,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Kumar Gala <galak@...eaurora.org>,
	Andy Gross <agross@...eaurora.org>,
	Sagar Dharia <sdharia@...eaurora.org>,
	Daniel Sneddon <dsneddon@...eaurora.org>
Subject: Re: [PATCH v2] spi: qup: Add DMA capabilities

On 02/24/2015 03:56 PM, Mark Brown wrote:
> On Tue, Feb 24, 2015 at 03:00:03PM +0200, Stanimir Varbanov wrote:
> 
>> +static void spi_qup_dma_done(void *data)
>> +{
>> +	struct spi_qup *qup = data;
>> +
>> +	if (atomic_dec_and_test(&qup->dma_outstanding))
>> +		complete(&qup->done);
>> +}
> 
> I'm finding it hard to be thrilled about the use of atomics for
> synchronization (they're just generally hard to work with) and...
> 
>> +	cookie = dmaengine_submit(desc);
>> +	ret = dma_submit_error(cookie);
>> +	if (ret)
>> +		return ret;
> 
>> +	atomic_inc(&qup->dma_outstanding);
> 
> ..don't we have two potential races here: one if somehow the DMA manages
> to complete prior to the atomic_inc() (unlikely but that's what race
> conditions are all about really) and one if we are issuing multiple DMAs
> and the early ones complete before the later ones are issued?
> 

yes, there is a potential race between atomic_inc and dma callback. I
reordered these calls to save few checks, and now it returns to me.

I imagine few options here:

 - reorder the dmaengine calls and atomic operations, i.e.
call atomic_inc for rx and tx channels before corresponding
dmaengine_submit and dmaengine_issue_pending.

 - have two different dma callbacks and two completions and waiting for
the two.

 - manage to receive only one dma callback, i.e. the last transfer in
case of presence of the rx_buf and tx_buf at the same time.

 - let me see for better solution.

Thanks for the comments.

regards,
Stan



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